isbn 0-321-19362-8 chapter 9 subprograms and functions –design issues –local referencing...

ISBN 0-321-19362-8

Chapter 9

Subprograms and Functions–Design Issues

–Local Referencing Environments

–Parameter-Passing Methods

–Parameters that are Subprogram Names

–Overloaded Subprograms

–Generic Subprograms

–User-Defined Overloaded Operators

–Coroutines

Introduction

• Two fundamental abstraction facilities– Process abstraction (this chapter)

• Procedures provide user-defined statements

• Functions provide user-defined operators

• Coroutines

– Data abstraction (Chapter 11)

Assumptions about Subprograms

• A subprogram has a single entry point

• The caller is suspended during execution of the called subprogram

• Control always returns to the caller when the called subprogram’s execution terminates

Some Definitions

• A subprogram definition is a description of the actions of the subprogram abstraction

• A subprogram call is an explicit request that the subprogram be executed

• A subprogram header is the first line of the definition, including the name, the kind of subprogram, and the formal parameters

• The parameter profile of a subprogram is the number, order, and types of its parameters

More Definitions

• The protocol of a subprogram is its parameter profile plus, if it is a function, its return type

• A subprogram declaration provides the protocol, but not the body, of the subprogram

• A formal parameter is a dummy variable listed in the subprogram header and used in the subprogram

• An actual parameter represents a value or address used in the subprogram call statement

Correspondence between Parameters and Arguments1. Positional2. Keyword

– e.g. SORT(LIST => A, LENGTH => N);– Advantage: order is irrelevant– Disadvantage: user must know the formal

parameter’s names– Default Values:e.g. procedure SORT(LIST : LIST_TYPE;

LENGTH : INTEGER := 100); ...SORT(LIST => A);

Design Issues for Subprograms

• What parameter passing methods are provided?• Are parameter types checked?• Are local variables static or dynamic?• Can subprogram definitions appear in other

subprogram definitions?• What is the referencing environment of a

passed subprogram?• Can subprograms be overloaded?• Are subprograms allowed to be generic?

Local referencing environments

• If local variables are stack-dynamic:– Advantages:

a. Support for recursion

b. Storage for locals is shared among some subprograms

– Disadvantages:a. Allocation/deallocation time

b. Indirect addressing

c. Subprograms cannot be history sensitive

• Static locals are the opposite

Implementations of Local Referencing Environments

1. FORTRAN 77 and 90 - most are static, but the implementor can choose either (User can force static with SAVE)

2. C - both (variables declared to be static are) (default is stack dynamic)

3. Pascal, Java, and Ada - dynamic only

Parameter Passing Methods

• We discuss these at several different levels:– Semantic models

• in mode• out mode• inout mode

– Conceptual models of transfer:

1. Physically move a value

2. Move an access path

Models of Parameter Passing

Implementation Models

1. Pass-by-value (in mode)

2. Pass-by-result (out mode)

3. Pass-by-value-result (inout mode)

4. Pass-by-reference (inout mode)

5. Pass-by-name (multiple mode)

Pass-by-value (in mode)

• Either by physical move or access path• Disadvantages of access path method:

– Must write-protect in the called subprogram

– Accesses cost more (indirect addressing)

• Disadvantages of physical move:– Requires more storage (duplicated space)

– Cost of the moves (if the parameter is large)

Pass-by-result (out mode)

• Local’s value is passed back to the caller

• Physical move is usually used

• Disadvantages:

– If value is passed, time and space

– In both cases, order dependence may be a problemprocedure sub1(y: int, z: int);

sub1(x, x);– Value of x in the caller depends on order of assignments at

the return

Pass-by-value-result (inout mode)

• Physical move, both ways

• Also called pass-by-copy

• Disadvantages:– Those of pass-by-result– Those of pass-by-value

Pass-by-reference (inout mode)

• Pass an access path• Also called pass-by-sharing• Advantage: passing process is efficient (no

copying and no duplicated storage)• Disadvantages:

– Slower accesses– Allows aliasing:

i. Actual parameter collisions:e.g. procedure sub1(a: int, b: int); ... sub1(x, x);

Pass-by-reference - disadvantages (cont)

ii. Array element collisions:

sub1(a[i], a[j]); /* if i = j */

Also, sub2(a, a[i]); (a different one)

iii. Collision between formals and globals

– Root cause of all of these is: The called subprogram is provided wider access to nonlocals than is necessary

– Pass-by-value-result does not allow these aliases (but has other problems!)

Pass-by-name (multiple mode)

• By textual substitution• Formals are bound to an access method at the

time of the call, but actual binding to a value or address takes place at the time of a reference or assignment

• Purpose: flexibility of late binding• Disadvantages

– Very inefficient references

– Too tricky; hard to read and understand

Pass-by-name Semantics

• If actual is a scalar variable, it is pass-by-reference

• If actual is a constant expression, it is pass-by-value

• If actual is an array element, it is like nothing elsesub1(i, a[i]);

• If actual is an expression with a reference to a variable that is also accessible in the program, it is also like nothing elsesub2(k+1, j, i); // where k is global

procedure sub1(x: int; y: int);

x := 1;

y := x;

x := 5;

y := x;

end;• If x is i and y is a[i], the two assignments to y will modify different elements of the array

procedure sub2(x: int; y: int; z: int);

k := 1; // k is global

y := x;

k := 5;

z := x;

end;• If the argument used for x is k+1, that expression will get re-evaluated each time x is used.

In Practice

• FORTRAN started out using pass-by-reference, later allowed pass-by-value-result

• ALGOL 60 used pass-by-name by default with pass-by-value optional

• C uses pass-by-value; can get pass-by-reference using pointers

• Pascal, Modula-2 and C++ use pass-by-value by default; pass-by-reference is optional

• Ada provides all three semantic modes • Java uses pass-by-value exclusively but use of

object references results in pass-by-reference behavior of objects

Type checking parameters

• Considered very important for reliability– FORTRAN 77 and original C: none

– Pascal, FORTRAN 90, Java, and Ada: it is always required

– ANSI C and C++: choice is made by the user

Implementing Parameter Passing• ALGOL 60 and most of its descendants use the run-

time stack• Value - copy it to the stack; references are indirect to

the stack• Result - same• Reference - regardless of form, put the address in the

stack• Name - run-time resident code segments or

subprograms evaluate the address of the parameter; called for each reference to the formal; these are called thunks– Very expensive, compared to reference or value-result

Stack Implementation of Parameter-Passing

Ada Parameter Passing

• Simple variables are passed by copy (value-result)

• Structured types can be either by copy or reference

• This can be a problem, because a) Of aliases (reference allows aliases, but value-

result does not)

b) Procedure termination by error can produce different actual parameter results

– Programs with such errors are “erroneous”

Design Considerations for Parameter Passing1. Efficiency2. One-way or two-way• These two are in conflict with one another!• Good programming => limited access to

variables, which means one-way whenever possible

• Efficiency => pass by reference is fastest way to pass structures of significant size

• Also, functions should not allow reference parameters

Overloaded Subprograms

• An overloaded subprogram is one that has the same name as another subprogram in the same referencing environment

• C++ and Ada have overloaded subprograms built-in, and users can write their own overloaded subprograms

Generic Subprograms

• A generic or polymorphic subprogram is one that takes parameters of different types on different activations

• Overloaded subprograms provide ad hoc polymorphism

• A subprogram that takes a generic parameter that is used in a type expression that describes the type of the parameters of the subprogram provides parametric polymorphism

Generic Subprograms

• C++ has templated functions (and classes)template <class Type>

Type max(Type first, Type second) { return first > second ? first : second; }

• C++ template functions are instantiated implicitly when the function is named in a call or when its address is taken with the & operatorint i, j;double x, y;…int k = max( i, j);double z = max( x, y);

Design Issues for Functions

1. Are side effects allowed?a. Two-way parameters (Ada does not allow)

b. Nonlocal reference (all allow)

2. What types of return values are allowed?– FORTRAN, Pascal - only simple types

– C - any type except functions and arrays

– Ada - any type (but subprograms are not types)

– C++ and Java - like C, but also allow classes to be returned

Accessing Nonlocal Environments

• The nonlocal variables of a subprogram are those that are visible but not declared in the subprogram

• Global variables are those that may be visible in all of the subprograms of a program

Nonlocal Environment Access

1. FORTRAN COMMON blocks– The only way in pre-90 FORTRANs to access nonlocal

variables– Can be used to share data or share storage

2. Static scoping - discussed in Chapter 53. External declarations - C

– Subprograms are not nested– Globals are created by external declarations (they are simply

defined outside any function)– Access is by either implicit or explicit declaration– Declarations (not definitions) give types to externally

defined variables (and say they are defined elsewhere)

4. External modules - Ada– More about these later (Chapter 11)

5. Dynamic Scope - discussed in Chapter 5

User-Defined Overloaded Operators• Nearly all programming languages have

overloaded operators• Users can further overload operators in C++

and Ada (Not carried over into Java)

Coroutines

• A coroutine is a subprogram that has multiple entries and controls them itself

• Also called symmetric control• A coroutine call is named a resume• The first resume of a coroutine is to its beginning, but

subsequent calls enter at the point just after the last executed statement in the coroutine

• Typically, coroutines repeatedly resume each other, possibly forever

• Coroutines provide quasi-concurrent execution of program units (the coroutines)

• Their execution is interleaved, but not overlapped

Coroutines

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